The above process can currently only be carried out at atmospheric pressure because the synthesis under pressure results in a thermally unstable product. However, the current process at atmospheric pressure has the disadvantage of using a larger excess of CH.sub.3 --Cl than if the synthesis had been carried out under pressure, the direct consequence of which is a greater gaseous discharge of CH.sub.3 --Cl.
Moreover, a process for the preparation of the salt (I) under pressure is known from Japanese Patent Application J 5 76-103 185. In accordance with this known process, a 50-80% (w/w) aqueous solution of a (meth)acrylate of formula: ##STR3## in which: Y represents a lower alkylene or lower hydroxy-alkylene group; and
R.sup.1 and R.sup.2 each independently represent a lower alkyl group PA1 (a) 5 to 15% by weight of the amount of CH.sub.3 --Cl necessary for the reaction are continuously introduced, at a temperature of 40-55.degree. C., into a closed reactor which contains the N,N-dimethylaminoethyl methacrylate and the pentasodium salt of diethylenetriaminepentaacetic acid and which has been pressurized with air from 1 to 3 bar; PA1 (b) water and the remainder of the CH.sub.3 --Cl are then continuously added, at a temperature of 45-55.degree. C., until the desired concentration of salt (I) in the water is obtained, it being possible for the pressure at the end of the reaction to reach 6-9 bar; then PA1 (c) the reactor is depressurized and, after returning to atmospheric pressure, the residual CH.sub.3 --Cl is removed, for example by stripping with air. PA1 DAMEMA: dimethylaminoethyl methacrylate PA1 Madquat MC 75: 75% aqueous methacryloyloxyethyltri-methylammonium chloride solution PA1 HQME: hydroquinone methyl ether PA1 Versenex 80: pentasodium salt of diethylenetriaminepentaacetic acid.
is brought into contact with an alkyl or alkaryl halide in order to quaternize the above methacrylate. The only example illustrating this quaternization is carried out with addition of methyl chloride under a maximum pressure of 4.5 bar, at a temperature of 25-30.degree. C. Besides the disadvantage, indicated above, relating to the discharge of the excess methyl chloride, this process results in significant hydrolysis of the starting compound, for example of N,N-dimethylaminoethyl methacrylate to methacrylic acid, this side reaction being promoted by the mode of introduction of the N,N-dimethylaminoethyl methacrylate and of the water, which are added already mixed and at the beginning of the reaction. This presence of methacrylic acid in the final aqueous solution obtained damages the analytical quality of the latter and can, in certain applications, have a harmful effect on its value for use in polymerization.
The aim of the present invention is to overcome the abovementioned disadvantages.
It has now been discovered, surprisingly, that the process under pressure could result in a thermally stable product, provided that it is carried out in the presence of a specific compound.